Pipeline mandrel positioning control system

ABSTRACT

A method and apparatus is provided for automated control and positioning of a mandrel within a pipe during bending operations. Position detection is accomplished by means of a low frequency electromagnetic signal transmission from a coil placed in close proximity parallel to the pipe outer surface. This signal is detected by a pair of orthogonal receiving coils mounted on the mandrel in close proximity to the pipe inner surface. The phases of the received signals with respect to the transmitted signal and the ratio of the amplitudes of the two received signals is used to estimate the relative position of the transmitter and the receivers. Control of the mandrel along with transmission of reference phase information is accomplished via a high frequency direct sequence spread spectrum radio link between a computer console outside the pipe mounted on the bender and a computer unit mounted inside the pipe on the mandrel.

TECHNICAL FIELD OF THE INVENTION

The current invention relates to mandrels used for bending largediameter pipes. In one aspect, it relates to a control system forautomatically positioning the mandrel within the pipe during bendingoperations.

BACKGROUND OF THE INVENTION

It is well known to utilize a mandrel positioned within a pipe toprovide internal support to the pipe walls and thereby prevent bucklingof the walls during bending of the pipe. Mandrels used for bending largediameter steel pipes of the type used for oil and gas pipelines (i.e.,where the pipe diameter can exceed 48 inches and the pipe wall thicknesscan exceed one inch) are very large pieces of equipment which can weighmany tons. Such mandrels typically have powered wheels or treads whichcan be remotely controlled to facilitate the longitudinal movement ofthe mandrel within the pipe.

When bending large diameter pipe to create a curved section, it iscommon practice to perform a succession of small discrete bends atlongitudinally spaced positions along the pipe. After each bend, thepipe is moved longitudinally through the bending apparatus until thelocation for the next desired bend is at the bending station. Themandrel must also be repositioned inside the pipe after each bend toprovide support at the next bend position. During the course of suchbending operations, the mandrel can be located fifty feet or more fromthe pipe end, and it is often out of sight of the person controlling it.Nevertheless, to achieve optimum results the mandrel must be accuratelypositioned with respect to the bending station, preferably within oneinch of the desired location, when each bend is performed.

Typically, large mandrels are positioned within the pipe under thecontrol of a dedicated mandrel operator who remains at the end of thepipe and sends movement commands to the mandrel by means of anelectrical cable or similar direct control device. However, the mandreloperator typically has no way to directly determine the position of themandrel with respect to the bending station. Instead, indirectmeasurements means must be used, such as determining the position of thepipe end relative to the bending station and then determining theposition of the mandrel relative to the pipe end. The latterdetermination is often accomplished using a reach rod, i.e., a rigidpole of known length connected to the end of the mandrel and extendingfrom the end of the pipe. After determining the estimated position ofthe mandrel with respect to the bending station, the operator sendsmovement commands to the mandrel until it has been moved into thedesired position.

The use of a dedicated mandrel operator for positioning a mandrel withinthe pipe has numerous disadvantages. First, employing a dedicatedmandrel operator represents a considerable expense to the pipelinecontractor. Second, the positioning accuracy of the mandrel is dependentupon the skill and care of the mandrel operator and is subject tosignificant deviations caused by human error. Third, the speed of thebending operation is highly dependent on the skill and experience of themandrel operator. For example, the weight of the mandrel results inconsiderable inertia which must be accounted for during movement,otherwise, the mandrel will "overshoot" the desired position. This canresult in a time consuming series of back-and-forth movements each timethe mandrel is repositioned. Finally, manual control systems require thehuman operator to stand along the line of motion of the heavy movingmandrel and pipe. A need therefore exists, for a system which cancontrol the position of a mandrel within a pipeline without requiring adedicated mandrel operator.

In view of the disadvantages inherent with manual control of themandrel, systems for automatically positioning a mandrel within a pipehave been proposed. For example, U.S. Pat. No. 5,651,638 to Heggeruddiscloses an apparatus for controlling the position and operation ofequipment within a pipeline. The Heggerud patent discloses andelectromagnetic communication system including transmitting andreceiving antenna external to the pipe for transmitting and receivingsignals through the wall of the pipe to and from, respectively,receiving and transmitting antenna mounted on equipment within the pipe.The Heggerud patent discloses one approach to controlling the positionof a mandrel within a pipeline, however, a need exists for alternativesystems for automatically controlling the position of a mandrel within apipeline.

SUMMARY OF THE INVENTION

The present invention is a method and apparatus for both manual andautomated control and positioning of a mandrel within a pipe duringbending operations. Position detection is accomplished by means of a lowfrequency electromagnetic signal transmission from a coil placed inclose proximity parallel to the pipe outer surface. This signal isdetected by a pair of orthogonal receiving coils mounted on the mandrelin close proximity to the pipe inner surface. The phases of the receivedsignals with respect to the transmitted signal and the ratio of theamplitudes of the two received signals is used to estimate the relativeposition of the transmitter and the receivers.

Control of the mandrel along with transmission of reference phaseinformation is accomplished via a high frequency direct sequence spreadspectrum radio link between a computer console outside the pipe mountedon the bender and a computer unit mounted inside the pipe on themandrel.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the invention and its advantages willbe apparent from the following detailed description when taken inconjunction with the accompanying drawings in which:

FIG. 1 is a partial cross-sectional view showing a pipeline mandrelposition control system according to one aspect of the currentinvention, including a mandrel control unit mounted on a mandrel withina pipe and a bender control unit positioned external to the pipe;

FIG. 2 is a partial cross-sectional view showing the relativeorientation of the transmit coil and the receive coil on opposite sidesof the pipe wall;

FIG. 3a is a graph of the amplitude of the signal produced by theperpendicular receive coil as a function of the relative longitudinalposition of the perpendicular receive coil from the longitudinal centerof the transmit coil;

FIG. 3b shows a graph of the phase of the signal produced by theperpendicular receive coil as a function of the relative longitudinalposition of the perpendicular receive coil from the longitudinal centerof the transmit coil;

FIG. 3c shows a graph of the amplitude of the signal produced by theparallel receive coil as a function of the relative longitudinalposition of the longitudinal center of the parallel receive coil fromthe longitudinal center of the transmit coil;

FIG. 3d is a graph of the phase of the signal produced by the parallelreceive coil as a function of the relative longitudinal position of thelongitudinal center of the parallel receive coil from the longitudinalcenter of the transmit coil;

FIG. 4 shows a schematic block diagram of a bender control unitaccording to a preferred embodiment of the current invention;

FIG. 5 shows a schematic block diagram of a mandrel control unitaccording to a preferred embodiment of the current invention;

FIG. 6 shows a block diagram of the automated sequence of a events forone bend operation according to another aspect of the current invention;and

FIG. 7 shows the control panel layout for the bender control unit.

DETAILED DESCRIPTION

Referring now to the drawings, wherein like reference numbers are usedto designate like elements throughout the various views, several aspectsof the current invention are further described. Referring first to FIG.1, shown therein is a mandrel 20 of a type well known in the art whichis suitable for use in one aspect of the current invention. The mandrel20 is positioned within a pipe 22 and includes upper and lower springunits 24, also known as shoes, which can be extended into engagementwith the interior surface of pipe walls 26 to support the walls duringthe bending operation. After bending, the shoes 24 can be retracted toallow the mandrel 20 to move longitudinally through the pipe 22 usingwheels 28. Power for movement of the shoes 24 and operation of thewheels 28 is supplied by engine unit 30.

To provided automatic control of the position of the mandrel 20 withinthe pipe 22, this aspect of the current invention includes a bendercontrol unit ("BCU") 32 mounted external to the pipe on the bendingapparatus (not shown) and a mandrel control unit ("MCU") 34 mounted onthe mandrel 20. The BCU 32 comprises a transmit coil 36, a directsequence spread spectrum ("DSS-type") radio frequency modem 38, and acomputer control unit 40. The MCU 34 comprises two receive coils 42 and44, a DSS-type radio frequency modem 46, and a computer control unit 48.Coils having a length of about 9 inches and a diameter of about 4 incheshave proven suitable for use as transmit and receive coils 36, 42 and44, however other size coils known in the art for low frequencyelectromagnetic communication can be used.

Referring now also to FIG. 2, the relative orientation of the transmitcoil 36 and the receive coils 42 and 44 will be further described. Thetransmit coil 36 is mounted on the bender unit (not shown) outside ofthe pipe wall 26 in close proximity to the external surface of the pipe22. The longitudinal axis of the transmit coil 36 is oriented parallelto the longitudinal axis 50 of the pipe 22. The receive coils 42 and 44are mounted on the mandrel inside the pipe 22. One of the receive coils,parallel receive coil 42, is mounted with its longitudinal axis orientedparallel to the longitudinal axis 50 of the pipe 22. The other receivecoil, perpendicular receive coil 44, is mounted with its longitudinalaxis oriented perpendicular to the longitudinal axis 50 of the pipe 22.The receive coils 42, 44 are mounted such that when the mandrel shoes 24are in the expanded position the coils are in close proximity to theinner surface of pipe wall 26.

As further described below, the BCU 32 and MCU 34 of the currentinvention utilize one-way transmission of low frequency electromagneticsignals transmitted through the pipe wall barrier in combination withtwo-way transmission of high frequency digital data transmitted aroundthe pipe wall barrier to provide very accurate detection of a mandrel'sposition within the pipe and to further to provide for automated orremote manual control of a mandrel within a pipe.

Two-way digital communication between the BCU 32 and the MCU 34 isprovided by the DSS-type radio frequency modems 38, 46 which transmitthrough antenna 52 and 54, respectively. The transmissions betweenantenna 52 and 54 (denoted generally in FIGS. 1 and 2 by dashed line 56)do not pass through the pipe wall barrier but instead are reflectedaround the pipe wall 26 by objects (denoted generally in FIG. 1 by tree58) in the surrounding environment, for example terrain, building,vehicles, vegetation, etc. A frequency of about 2.4 GHz is known to beeffective for DSS communication between the BCU 32 and the MCU 34. TheDSS-type communication technique, which is well known in the art, hasthe advantage of improved reliability under multi-path interferenceconditions and the high frequency of operation permits operation withsmall pipe diameters. A radio link range of about 0.66 miles has beendemonstrated with the equipment of the embodiment shown. Since thetypical anticipated operation range for the equipment is a few hundredfeet, reliability is greatly enhanced.

A constant flow of two-way messages (50 or more per second) istransmitted in the form of command packets from the BCU 32 to the MCU 34and corresponding response packets sent from the MCU back to the BCUupon receipt of each command packet. The BCU command packets includemandrel control information and timing update information (describedbelow), while the MCU response packets include mandrel positioninformation and input status information. Loss of a valid communicationstream (i.e., command packets not received by the MCU 34 for apredetermined time or expected response packets not received from theMCU by the BCU 32 within a predetermined time after sending) will resultin the MCU 34 executing safety measures such as shutdown of hydrauliccontrols and eventual shutdown of the mandrel engine unit 30.

The BCU 32 records the send time for each command packet sent to the MCU34 and the receipt time for the corresponding response packet receivedfrom the MCU. The BCU 32 then calculates the round-trip ("RT")transmission time for each packet and maintains a running averageround-trip ("ART") time calculated from a predetermined number of themost recent RT times. The ART time is utilized in detecting the mandrelposition as further explained below. An ART time based on the 100 mostrecent RT times has been shown to provide satisfactory results.

As mentioned above, the one-way transmission of low frequencyelectromagnetic signals (denoted generally in FIGS. 1 and 2 by wave 60)transmitted through the pipe wall barrier is used in combination withthe two-way communication described above to determine the position ofthe mandrel 20 within the pipe 22. The BCU 32 generates a continuous lowfrequency sinusoidal electromagnetic signal 60 by exciting (via leads62) the transmit coil 36 mounted outside the pipe 22 in close proximityto the outer surface of the pipe. The frequency of this signal ispreferably within the range from about 21 Hz to about 24 Hz, and morepreferably is within the range from about 22 Hz to about 23 Hz. Signalfrequencies lower than about 21 Hz tend to encounter interference fromengine noise originating with the mandrel or other nearby equipment andknown to be strong at 18 Hz. Signal frequencies higher than 24 Hzrapidly lose the ability to penetrate the metal pipe wall barrier withsufficient strength to provide useful range. This signal 60 is detectedby a pair of receive coils 42, 44 mounted on the mandrel 20 inside thepipe 22 in close proximity to the inner surface of the pipe wall 26. Theamplitude and relative phase of these signals is used to estimate therelative position between the transmitting coil 36 and the receivingcoils 42, 44.

FIGS. 3a-3d shows plots of the amplitudes and phases of the signals onleads 64 and 66 received by the parallel receive coil 42 and theperpendicular receive coil 44, respectively, versus the relativeposition of the receive coils with respect to the transmit coil 36. Thesignal transmitted by the transmit coil 36 is supplied by leads 62. Forconvenience, the signal transmitted by the transmit coil 36, andreceived by the parallel receive coil 42 and perpendicular receive coil44, will hereafter be referred to using the reference numbers of thecorresponding leads, namely 62, 64 and 66, respectively.

FIG. 3a is a graph of the amplitude of the signal 66 received byperpendicular receive coil 44 versus the relative longitudinal positionof the perpendicular receive coil with respect to the center of thetransmit coil 36. As shown in FIG. 3a, the amplitude of theperpendicular receive signal 66 has a null 68 when the relative positionis zero (also called the "center" position, i.e., when the perpendicularreceive coil 44 is longitudinally centered with respect to the transmitcoil 36), increases to a local maximum value 70 on either side of thezero position, and then decays back toward zero as the distancesincrease to the right or left of the local maxima.

FIG. 3b is a graph of the phase of the signal 66 received by theperpendicular receive coil 44 versus the relative longitudinal positionof the perpendicular receive coil with respect to the center of thetransmit coil 36. The phase of signal 66 is measured with respect to thephase of the signal 62 transmitted by transmit coil 36. As shown in FIG.3b, the perpendicular receive signal 66 is in phase with the transmitcoil signal 62 for negative relative distances (i.e., left of the zeroposition) and is out of phase for positive relative distances (i.e.,right of the zero position).

FIG. 3c is a graph of the amplitude of the signal 64 received by theparallel receive coil 42 versus the relative longitudinal position ofthe center of the parallel receive coil with respect to the center ofthe transmit coil 36. As shown in FIG. 3c, the amplitude signal 64 has alocal maximum value 72 when the relative distance is zero (i.e., whenthe center of the parallel receive coil 42 is longitudinally centeredwith respect to the transmit coil 36), has nulls 74 at either side ofthe zero position, then has secondary maxima 76 as the distance from thezero position increases, and finally decays toward zero as the distanceincreases to the left and right of the secondary maxima 76. The zone 78between the nulls 74 is referred to as the "near zone" and the zones 80to the left and right of the nulls are referred to as the "far zone".

FIG. 3d is a graph of the phase of the signal 64 received by theparallel receive coil 42 versus the relative longitudinal position ofthe center of parallel receive coil with respect to the center of thetransmit coil 36. As before, the phase of signal 64 is measured withrespect to the phase of the signal 62 transmitted by transmit coil 36.As shown in FIG. 3d, the parallel receive signal 64 is in phase with thetransmit coil signal 62 in the far zones 80 and is out of phase withinthe near zone 78.

Although not required, it is definitely preferred that the receive coils42, 44 are mounted such that the perpendicular receive coil 44 islongitudinally centered with respect to the parallel receive coil 42, asshown in FIG. 2. Such a mounting arrangement will result in thelongitudinal correspondence of the zero/center positions for the tworeceive coils, greatly simplifying the interpretation of the amplitudeand phase relationships necessary to determine the position of thereceive coils with respect to the transmit coil 36 (and hence, theposition of the mandrel 20 with respect to the BCU 32).

Detection of relative phase of the received low frequency signals 64, 66requires knowledge of the phase of the transmitting signal 62. A replicaof the transmitted signal 62 is therefore maintained in the MCU 34 as alocal reference for phase computation purposes. This local reference iscontinuously adjusted to match the timing of the transmitting signal 62by means of information received over the DSS radio link 56. Since thereis an inherent and variable delay in the DSS radio channel due tosoftware buffering, error correction and channel hopping, the BCU 32accounts for these delays by keeping a history (e.g., in the form of arunning average or other statistical value) of the communication roundtrip times, i.e., the RT times. The reference phase informationtransmitted by the BCU 32 to the MCU 34 over the DSS channel is thenpre-adjusted by one-half the average round trip delay time (i.e., by0.5×ART) such that the phase information received by the MCU 34 willallow accurate determination of the phases of the received signals 64,66.

The MCU 34 amplifies, filters and digitizes the low frequencyelectromagnetic signals 64, 66 detected by the parallel andperpendicular receive coils 42, 44. The perpendicular receive coilsignal 66 is digitized and then cross correlated with various timeshifted versions of the local reference. The maximum cross correlationresult corresponds to the best match of the signals being correlated.The relative phase information between the transmit coil signal andperpendicular receive coil signal is thus determined. The parallel coilsignal 64 is also processed in a similar fashion to determine theparallel phase. The best correlation match also provides amplitudeinformation for both the receive coils 42, 44. The phase informationfrom both receive coils is used by computer control unit 48 to uniquelydetermine which zone (i.e., near zone left of center, near zone right ofcenter, far zone left of center, far zone right of center) of therelative position the mandrel occupies. The amplitude ratios (i.e.,between the two receive coil signals 64, 66) are used by the computercontrol unit 48 to determine the relative distance of the mandrel withineach zone. Since position estimation is based on amplitude ratios ratherthan absolute values, the estimation is robust and immune to smallvariations in signal strength and quality.

FIG. 4 shows a block diagram of a BCU 32 according to one embodiment ofthe current invention. The BCU 32 includes a digital signal processor("DSP") 82, a power driver 84 for energizing the transmit coil 36, andthe direct sequence spread spectrum radio modem 38. The DSP 82 isoperably connected to the power driver 84 by means of optical isolationunit 86. The DSS radio modem 38, which is operably connected to the DSP82 by means of an optical isolation unit 88 and a universal asynchronousreceiver/transmitter ("UART") 90, transmits and receives through antenna52. A user control panel 92 is operably connected to the DSP 82 by meansof optical isolation unit 96 for providing digital and analog inputs(denoted generally by reference numeral 94) to the DSP. Indicators 98are operably connected to the DSP 82 for receiving signals from the DSPindicating the status of the system components and the position of themandrel 20 within the pipe 22. The indicators 98 are preferably,although not necessarily, located on the control panel 92. Anasynchronous serial link 100, operably connected to the DSP 82 by meansof optical isolation unit 102 and UART 104, is provided for diagnosticsand maintenance, for example, to allow field software upgrades to beuploaded to the DSP.

FIG. 5 shows a block diagram of the MCU 34 according to one embodimentof the current invention. The MCU 34 includes a digital signal processor106, receive signal conditioning equipment, and a DSS radio modem 46.The signal 64 received by the parallel receive coil 42 is conditioned bya differential amplifier 108 and a band pass filter 110 before beingdigitized by an analog-to-digital converter 112 and delivered to DSP106. Similarly, the signal 66 received by the perpendicular receive coil44 is conditioned by a differential amplifier 114 and a band pass filter116 before being digitized by an analog-to-digital converter 118 anddelivered to the DSP 106. The DSS radio modem 48, which is operablyconnected to the DSP 106 by means of an optical isolation unit 120 and aUART 122, transmits and receives through antenna 54. Analog and digitaloutput signals (denoted generally by reference number 124) from the DSP106 pass through an optical isolation unit 126 for controlling mandrelfunctions such as extension and retraction of the shoes 24, operation ofthe wheels 28, and control of the engine power unit 30. In addition, DSP106 can activate relays 128 to control other functions on the mandrel.External control inputs and feedback signals from the control system(denoted generally by reference numeral 130) are fed into the DSP 106 bymeans of optical isolation unit 132. An asynchronous serial link 134,operably connected to the DSP 106 by means of optical isolation unit 136and UART 138, is provided for diagnostics and maintenance.

Pipe bending operations involve repeated precise position of the mandrel20 within the pipe 22. The control system of the current inventioneliminates the need for a dedicated mandrel operator to assist thebender apparatus operator by automating the positioning of the mandrelwithin the pipe. During automatic operation, a single command by theoperator at the BCU console 92 will cause the mandrel 20 to retract thesupporting shoes 24, move right or left (i.e., longitudinally) throughthe pipe 22 as necessary to center itself with respect to the transmitcoil 36 (e.g., to longitudinally center the receive coils 42, 44 withthe transmit coil 36) and then to extend the shoes into engagement withthe pipe wall 26 to provide support for the next bend operation.

During automated positioning of the mandrel 22 within the pipe 20, thecurrent mandrel position (with respect to the center position as definedby the transmit coil 36) as reported by the MCU 34 is used by the BCU tocompute the power drive applied to the hydraulic proportional valve (notshown) which controls the mandrel forward/reverse motion. Thiscomputation is preferably based on a conventionalProportional-Integral-Derivative (PID) control algorithm, however, otheralgorithms known in the art for positioning control can be used. Thisfeedback control is maintained until the mandrel stabilizes at thecenter position.

FIG. 6 provides a block diagram flow chart of the steps comprising anautomated bend operation according to another aspect of the currentinvention. Referring first to block 200, the automated bend procedurebegins with a check to determine whether the MCU 34 mounted on themandrel is within range of the BCU 32 mounted on the bending machine. Inthe preferred embodiment, the range check comprises determining if thereceived signal quality and strength of both the parallel receive coilsignal 64 and the perpendicular receive coil signal 66 are sufficient toreliably determine the relative positions between the transmit coil 36and the receive coils 42, 44: If the signals are sufficient, this isconsidered an "in range" condition, and the procedure advances to block202; while if either signal is not sufficient, an "out of range"condition exists and the bend operation is aborted to block 204. In thepreferred embodiment, an "in range" condition is indicated bycontinuously lighting a RANGE light 302 on the BCU control panel 92, andan "out of range" condition is indicated by blinking the RANGE light.Aborting the procedure to block 204 disables further automatic operationuntil an "in range" condition is established.

At block 202, the procedure waits until a NEXT PULL signal is receivedfrom the control panel 92. In the preferred embodiment, the NEXT PULLsignal is produced when the INCREMENT button is pushed on the controlpanel 92 during automatic mode of operation (selected by CONTROL MODEswitch 306). When the BCU 32 receives a NEXT PULL signal, the procedureadvances to block 206, wherein the BCU commands (via radio link 56) theMCU 34 to retract the shoes 24 of the mandrel. The procedure thenadvances to block 208, where the procedure checks to determine if theshoes 24 have been retracted. In the preferred embodiment, this check isperformed by monitoring low side pressure input to the shoe mechanism.If the check determines the shoes 24 are retracted (i.e., down), thenthe procedure advances to block 210, while if the check determines thatthe shoes are still extended (i.e., up), then the procedure advances toblock 212. Block 212 is simply a timing loop, i.e., if the automaticoperation takes more than a preset time limit, this is regarded as a"timeout" condition and the procedure aborts to block 214. Otherwise,the timing loop returns to block 208 to update the shoe position check.In the preferred embodiment, the position of the mandrel shoes 24 isindicated on the control panel 92 by the UP light 308 and the DOWN light310.

At block 210, the procedure checks to determine if the BCU 32 has validmandrel position value which has been received from the MCU 34. If so,the procedure advances to block 218, otherwise, the procedure aborts toblock 216. Aborting the procedure to block 216 disables furtherautomatic operation until an valid position is received.

At block 218, the BCU 32 transmits movement commands to the MCU 34 asnecessary to move the mandrel 20 to the center position (i.e., zeroposition) with respect to the transmit coil 36. As previously described,the parameters of the movement commands (e.g., direction, speed) areestablished by a control algorithm (e.g.,proportional-integral-derivative algorithm) using an estimate of thecurrent position reported by the MCU. The movement command resultingfrom the control algorithm is then transmitted by the BCU to the MCUover the radio link 56. Once received by the MCU, the movement commandcauses the MCU to produce digital or analog outputs which activate themandrel controls for forward and reverse motion. The mandrel's positionis continuously updated to the BCU over the radio link 56 and the newpositions are used as feedback in the control algorithm to adjustsubsequent movement commands. The feedback control continues until themandrel stabilizes at the center/zero position.

While the BCU is moving the mandrel toward the center/zero position, theprocedure has meanwhile advanced to block 220, where a check isperformed to determine whether the mandrel has achieved the centerposition. If so, the procedure advances to block 226, while if not, theprocedure enters a timing loop through block 222 and back to block 218.If the automatic centering operation takes more than a preset timelimit, this is regarded as a "timeout" condition and the procedureaborts to block 224.

At block 226, the BCU 32 commands (via radio link 56) the MCU 34 toextend the shoes 24 of the mandrel. The procedure then advances to block228, where the procedure checks to determine if the shoes 24 have beenextended. In the preferred embodiment, this check is performed bymonitoring high side pressure input to the shoe mechanism. If the checkdetermines the shoes 24 are extended (i.e., up), then the procedureadvances to block 230 (end of the bend procedure), while if the checkdetermines that the shoes are still retracted (i.e., down), then theprocedure advances to block 232. Block 232 is another timing loop. Ifthe automatic "shoes up" operation takes more than a preset time limit,this is regarded as a "timeout" condition and the procedure aborts toblock 234. Otherwise, the timing loop returns to block 228 to update theshoe position check.

In a preferred embodiment of the invention, the procedure includesperiodic checks for error conditions including loss of radiocommunication, poor signal quality or strength at the receive coils 42,44, excessive time required for automatic operation, emergency stop byBCU operator, manual override control inputs by BCU operator. Thedetermination that any of such error conditions exists causes prematuretermination of the automatic mode of operation and/or shutdown ofcontrols for safety purposes. Further in the preferred embodiment,short-term error conditions trigger the shutdown of hydraulic controlsignals for shoes up/down and forward/reverse motion of the mandrel. Ifthe error conditions persist for more than a pre-determined time, themandrel engine is shutdown for safety purposes.

Referring now to FIG. 7, the control layout of the BCU control panel 92for the preferred embodiment is shown. Numerous indicators of systemstatus are provided, including a COM light 312 (lit when radio linkoperating), the RANGE light 302 (lit when valid position informationavailable), an AUTO light 314 (lit to indicate automatic mode enabled),a LEFT light 316 (lit to show when mandrel detected passing the centerposition from right to left), a RIGHT light 318 (lit to show whenmandrel detected passing the center position from left to right), the UPlight 308 and the DOWN light 310 lit to show the position of the mandrelshoes 24. Switches are provided to activate various system functions,including SYSTEM switch 322 (turns system on/off), CONTROL MODE switch306 (selects automatic/manual mode), ENGINE START switch 324 (turnsmandrel engine on), ENGINE STOP switch 326 (turns mandrel engine off),and INCREMENT switch 304 (advances pipe in bender to next bendingposition, and if in automatic mode, initiates automatic mandrelcentering procedure). A control joystick 328 is provided, allowing thebender operator to remotely control the mandrel movement(forward/reverse, i.e., right/left with respect to zero position) andshoe position as necessary. Finally, an emergency switch 330 is providedto shut down the system.

While several aspects and embodiments of the current invention have beendescribed in detail herein, it will be readily apparent that manychanges in detail may be made as a matter of design choices, withoutdeparting from the spirit and scope of the invention, as defined by theappended claims.

We claim:
 1. An apparatus for controlling the position of a mandrelwithin a pipe comprising:an exterior control unit including a lowfrequency signal generator; a transmit coil, and a first radio frequencytransceiver;said transmit coil being positioned proximate to the outsidesurface of a wall of said pipe and having a longitudinal axis orientedparallel to the longitudinal axis of the pipe; said signal generatorbeing operably connected to said transmit coil to transmit low frequencyelectromagnetic signals through said wall; said radio frequencytransceiver transmitting a radio frequency signal; an interior controlunit mounted on said mandrel and including a first receive coil, asecond receive coil and a second radio frequency transceiver;said firstreceive coil having a longitudinal axis oriented parallel to thelongitudinal axis of the pipe, said coil being adapted to receive saidlow frequency electromagnetic signals passing through said wall; saidsecond receive coil having a longitudinal axis oriented perpendicular tothe longitudinal axis of the pipe, said coil being adapted to receivesaid low frequency electromagnetic signals passing through said wall;said second radio frequency transceiver adapted to receive said radiofrequency signal containing information regarding the phase of said lowfrequency signal; anda digital signal processor adapted to compare thephase of signals received by said first and said second receive coils toa time base containing information regarding the phase of said lowfrequency signal received from said first radio frequency transceiver.2. An apparatus for controlling the position of a mandrel within a pipeaccording to claim 1, wherein said digital signal processor is furtheradapted to compare the amplitude of the signal received by one of saidfirst and second receive coils to the amplitude of the signal receivedby another of said first and second receive coils.
 3. An apparatus forcontrollling the position of a mandrel within a pipe according to claim1, wherein said first radio frequency transceiver is a direct sequencespread spectrum radio frequency modem.
 4. An apparatus for controllingthe position of a mandrel with a pipe according to claim 3, wherein saidsecond radio frequency transceiver is a direct sequence spread spectrumradio frequency modem.
 5. An apparatus for controlling the position of amandrel within a pipe according to claim 3, wherein said radio frequencysignal has a frequency of about 2.4 GHz.
 6. An apparatus for controllingthe position of a mandrel within a pipe according to claim 1, whereinsaid low frequency electromagnetic signals have a frequency within therange from about 21 Hz to about 24 Hz.
 7. An apparatus for controllingthe position of a mandrel within a pipe according to claim 6, whereinsaid low frequency electromagnetic signals have a frequency within therange from about 22 Hz to about 23 Hz.
 8. An apparatus for controllingthe position of a mandrel within a pipe according to claim 1, whereinsaid first and second receive coils are mounted such that said secondreceive coil is longitudinally cnertered with respect to said firstreceive coil.
 9. An apparatus for controlling the position of a mandrelwithin a pipe according to claim 1, wherein said interior control unitis adapted to maintain a replica of said low frequency signaltransmitted by said exterior control unit, said replica beingcontinuously adjusted to match the timing of said low frequency signalusing information received from saiid exterior control unit by saidsecond radio frequency transceiver.
 10. An apparatus for controlling theposition of a mandrel within a pipe according to claim 1, wherein saidradio frequency signal transmitted by said first radio frequencytransceiver includes a plurality of command data packets, said interiorcontrol unit is adapted to receive said command data packets and totransmit an answering radio frequency signal including a correspondingresponse data packet after receiving each of said command data packets,and said exterior control unit is adapted to receive said answeringradio frequency signal.
 11. An apparatus for controlling the position ofa mandrel within a pipe according to claim 10, wherein said command datapackets comprise said information regarding the phase of said lowfrequency signal.
 12. An apparatus for controlling the position of amandrel within a pipe according to claim 10, wherein said exteriorcontrol unit is adapted to calculate a round-trip transmission time foreach of said command data packets by first recording a send time whenone of said command data packets is transmitted, then recording areceipt time when a response data package corresponding to said one ofsaid command data packets is received by said exterior control unit, andthen determining the time difference between said send time and saidreceipt time, said time difference being said round-trip transmissiontime.
 13. An apparatus for controlling the position of a mandrel withina pipe according to claim 12, wherein said exterior control unit isadapted to maintain a running average round-trip time, said runningaverage round-trip time being calculated by taking the arithmeticaverage of a predetermined number of the most recent of said round-triptransmission times.
 14. An apparatus for controlling the position of amandrel within a pipe according to claim 13, wherein said predeterminednumber of the most recent of said round-trip transmission times used forcalculating said running average round-trip time is
 100. 15. Anapparatus for controlling the position of a mandrel within a pipeaccording to claim 13, wherein time base information regarding the phaseof said low frequency signal is pre-adjusted by an amount equal to 0.5times the running average round-trip time before being transmitted bysaid exterior control unit over said radio frequency signal.
 16. Anapparatus for controlling the position of a mandrel within a pipecomprising:a first control unit including a transmit coil and a firstradio frequency transceiver;said transmit coil being positionableproximate to one of an inside surface and an outside surface of a wallof said pipe and being adapted to transmit low frequency signals throughsaid wall; said first radio frequency transceiver being adapted totransmit radio signals containing information regarding the phase ofaaid low frequency signal; a second control unit including a firstreceive coil, a second receive coil and a second radio frequencytransceiver;said first receive coil being positionable proximate to another of said inside surface and said outside surface of said wall ofsaid pipe, having a first orientation with respedt to the longitudinalaxis of said pipe, and being adapted to receive said low frequencysignals passing through said wall; said second receive coil beingpositionable proximate to an other of said inside surface and saidoutside surface of said wall of said pipe, having a second orientationwith respect to the longitudinal axis of said pipe, and being adapted toreceive said low frequency signals passing through said wall; saidsecond radio frequency transceiver being adapted to receive said radiosignals containing information regarding the phase of said low frequencysignals; and a digital signal processor adapted to compare the phase oflow frequency signals received by said first and said second receivecoils to a time base containing information regarding the phase of saidlow frequency signal received from said first radio frequencytransceiver.
 17. An apparatus for controlling the position of a mandrelwithin a pipe according to claim 16, wherein said digital signalprocessor is further adapted to compare the amplitude of said signalreceived by one of said first and second receive coils to the amplitudeof the signal received by another of said first and second receivecoils.
 18. An apparatus for controlling the position of a mandrel withina pipe according to claim 16, wherein said sescond control unit isadapted to maintain a replica of said low frequency signal transmittedby said first control unit, said replica being continuously adjusted tomatch the timing of said low frequency signal using information receivedfrom said first control unit by said second radio frequency tansceiver.19. An apparatus for controlling the position of a mandrel within a pipeaccording to claim 16, wherein said radio signal transmitted by saidfirst radio frequency transceiver includes a plurality of command datapackets, said second control unit is adapted to receive said commanddata packets and to transmit an answering radio signal including acorresponding ressponse data packet after receiving each of said commanddata packets, and said first control unit is adapted to receive saidanswering radio signal.
 20. An apparatus for controlling the position ofa mandrel within a pipe according to claim 19, wherein said firstcontrol unit is adapted to calculate a round-trip transmission time foreach of said command data packets by first recording a send time whenone of said command data packets is transmitted, then recording areceipt time when a response data package corresponding to said one ofsaid command data packets is received by said first control unit, andthen determining the time difference between said send time and saidreceipt time, said time difference being said round-trip transmissiontime, and said first control unit is further adapted to use saidround-trip transmission time to adjust time base information regardingthe phase of said low frequency signal before transmitting said timebase information over said radio signal.